The present invention relates to power supplies, and more particularly to a voltage regulator with thermal overload protection for use with electronic flash units.
Numerous types of electronic photoflash units currently are available, including manual and automatic types. In a typical automatic electronic photoflash unit, a relatively large main capacitor is used as the power source for the electronic flash tube. With the main capacitor suitably charged, the flash tube can be triggered, whereupon energy from the capacitor is dumped into the flash tube and causes the flash tube to emit light. In the automatic type unit, a light sensor circuit is provided which senses the light reflected from the subject being photographed, and when sufficient light has been received this circuit operates to extinguish light emission from the flash tube. In one class of electronic photoflash unit of the automatic type, a low resistance path is applied across the flash tube to thereby shunt the remaining capacitor energy therethrough. In another type, a switch is used in series with the flash tube, and when sufficient light has been received, this switch is turned off to thereby extinguish the flash of light.
Typical flash units of this nature include a builtin power supply with either disposable or rechargable batteries, along with an appropriate converter circuit for supplying a charging voltage to the main capacitor, which voltage typically is of the order of 330 volts dc. An example of a prior photoflash power supply using a series regulator between a dc source and main capacitor of a flash unit is shown in U.S. Pat. No. 3,819,893.
The usual rechargeable batteries have sufficient energy for exposing a roll or so of film, and then need to be recharged. Although present-day electronic flash units sometimes have a rapid charge capability for the rechargeable battery system, there are instances when photographers desire a portable power supply or battery pack which can be connected with the electronic flash unit, and which will provide the capability for more flash exposures. While such battery packs provide this capability, and the capability to rapidly recharge the main capacitor of the flash unit, they can damage the flash unit. When the photographer, using such a battery pack, takes a rapid sequence of pictures, because of the number of times the main capacitor of the flash unit is recharged over a short period of time, damage to the electronic flash unit may result easily.